The present invention relates to a process for producing 2,2'-bis(hydroxymethyl)alkanal and 2,2'-bis(hydroxymethyl)alkanoic acid. More particularly, the present invention relates to a process for producing 2,2'-bis(hydroxymethyl)alkanal and 2,2'-bis(hydroxymethyl)alkanoic acid as raw materials of dimethylol alkanoic acids or trimethylol alkanes, which are useful raw materials for the production of polyesters, polyurethanes, alkyd resins or the like, in an industrially advantageous manner.
It is well known in the art that 2,2'-bis(hydroxymethyl)alkanals have been produced by reacting aliphatic aldehyde with formaldehyde in the presence of a basic substance.
As the basic substances used in the above-mentioned production of 2,2'-bis(hydroxymethyl)alkanals, there have been proposed various substances such as sodium hydroxide (Japanese Patent Publication (KOKOKU) No. 52-20965(1977) and Japanese Patent Application Laid-open (KOKAI) No. 62-263141(1987)), sodium carbonate (U.S. Pat. No. 3,312,736), triethylamine (Japanese Patent Publication (KOKOKU) No. 4-55181(1992)) or dimethylamino neopentanol (German Patent No. 2507461).
In these conventional processes, since formaldehyde is used in approximately stoichiometric amount, i.e., in such an amount that the molar ratio of formaldehyde to aliphatic aldehyde is about 2, a considerable amount of 2-substituted acrolein is produced as a by-product, thereby causing difficulty in producing the desired 2,2'-bis(hydroxymethyl)alkanals in industrially advantageous.
On the contrary, when formaldehyde is used in such an increased amount of not less than 10 equivalents based on an equivalent amount of aliphatic aldehyde in order to inhibit the production of 2-substituted acrolein as a by-product, the desired 2,2'-bis(hydroxymethyl)alkanals can be produced with a high yield. However, in this case, an excess amount of formaldehyde remains unreacted, so that it is necessary to subsequently conduct complicated procedures for removal or recycling of the excess amount of residual formaldehyde, resulting in an increase in the production cost. Further, in the case where such an excess amount of formaldehyde remains in the alkanal as the desired product, there also arises the problem that a large amount of expensive oxidizing agent must be added to the reaction mixture in the subsequent oxidation step or otherwise an undesirable side reaction will occur therein.
That is, when aliphatic aldehyde and formaldehyde are charged in a molar ratio of 1:2 to 1:10 and reacted with each other in the presence of a base catalyst, a considerable amount of 2-substituted acrolein is disadvantageously produced as a by-product together with the desired 2,2'-bis(hydroxymethyl)alkanal. The amount of 2-substituted acrolein produced as a by-product is varied depending upon the kind of aliphatic aldehyde used, the kind and amount of base catalyst used, the reaction temperature or the like. For instances, when n-butyl aldehyde is reacted with formaldehyde at a temperature of about 60.degree. C. in the presence of triethylamine, 2-ethyl acrolein is produced in an amount as large as about 20 mole %. It is also known that the 2-substituted acrolein is produced by dehydration of 2-hydroxymethylalkanal which is a precursor of 2,2'-bis(hydroxymethyl)alkanal, and there exists equilibrium between 2-hydroxymethylalkanal and the 2-substituted acrolein. Therefore, where the molar ratio of aliphatic aldehyde to formaldehyde charged is small, the production of the 2-substituted acrolein as a by-product cannot be avoided.
On the other hand, the 2-substituted acrolein can be converted into 2,2'-bis(hydroxymethyl)alkanal as the desired product by reacting with formaldehyde in the presence of a base catalyst and water. However, in order to obtain the desired product with a high yield, it is required to use formaldehyde in an excess amount of from 10 to 30 moles per mole of the 2-substituted acrolein. As described above, in this case, a large amount of formaldehyde remains unreacted, so that additional steps are required to separate the residual formaldehyde from the desired product, resulting in an industrially disadvantageous process.
In addition, as methods for converting the 2-substituted acrolein by-product into 2,2'-bis(hydroxymethyl)alkanal, include those various methods mentioned below. In Japanese Patent Application Laid-open (KOKAI) No. 52-124213(1977), there has been proposed a method of producing 2,2'-bis(hydroxymethyl)butanal by reacting 2-ethyl acrolein with an aqueous formaldehyde solution in the presence of triethylamine. However, in this method, a considerable excess amount of formaldehyde relative to 2-ethyl acrolein is requires thus resulting in an increase in production cost.
In German Patent No. 2507461 (British Patent No. 1,535,826), there has been proposed a process which comprises a first step of reacting n-butyl aldehyde with formaldehyde in a reactor, for example, in the presence of N,N-dimethylamino neopentanol, followed by removing unreacted n-butyl aldehyde and 2-ethyl acrolein as a by-product from the reaction solution by distillation, and a second step of adding formaldehyde and amine to the resultant distillate to react with each other. However, the German Patent is wholly silent about not only control or adjustment of the molar ratio between the 2-substituted acrolein and formaldehyde in the reaction system, but also control or adjustment of the molar ratio between aliphatic aldehyde and formaldehyde. Further, in the process of the German Patent, formaldehyde is used in approximately stoichiometric amount, resulting in low yield of the desired product as described hereinbefore. This also indicates that the process of the German Patent is not industrially advantageous.
Furthermore, it is known in the art that 2,2'-bis(hydroxymethyl)alkanal is converted into dimethylol alkanoic acid by oxidation and into trimethylolalkane by hydrogenation.
As the methods of producing 2,2'-bis(hydroxymethyl)alkanoic acid by the oxidation of 2,2'-bis(hydroxymethyl)alkanal, there have been proposed a method of conducting the oxidation using hydrogen peroxide (e.g., U.S. Pat. No. 3,312,736), a method of conducting the oxidation in the presence of at least one compound catalyst selected from the group consisting of cerium, titanium, zirconium, tin, niobium, molybdenum and tungsten using hydrogen peroxide (Japanese Patent Application Laid-open (KOKAI) 62-263141(1987)) and a method of conducting the oxidation using perisobutyric acid (Journal of Institute of Organic Synthesis Chemistry, 36, 1095(1978)), or the like. However, in these conventional methods in which 2,2'-bis(hydroxymethyl)alkanal produced by known methods is subjected to oxidation, there arise problems that the yield of 2,2'-bis(hydroxymethyl)alkanal is low, and that since a large amount of residual formaldehyde is contained in the reaction solution, it is necessary to add a large amount of an oxidizing agent thereto in the subsequent oxidation step. As a result, it becomes impossible to obtain the desired product with a high purity and a high yield.
That is, heretofore methods capable of producing 2,2'-bis(hydroxymethyl)alkanal with a high yield, and reducing the amount of residual formaldehyde in the reaction solution have been unknown.
It has now been discovered herein that by reacting aliphatic aldehyde with formaldehyde in the presence of a base catalyst and reacting the resultant 2-substituted acrolein by-product with a specific amount (molar ratio) of formaldehyde in the presence of a base catalyst while controlling or adjusting amounts (molar ratio) of aliphatic aldehyde and formaldehyde to be charged to within specified values, the amount of residual formaldehyde in the reaction solution can be considerably reduced and the desired 2,2'-bis(hydroxymethyl)alkanal can be produced with a high yield in an industrially advantageous manner.